How can I brew lager at room temperature?

I think 30°C(86°F) will be far too hot for a lager, even for a California common yeast such as Wyeast 2112 with a max temp of 20°C(68°F).

I know it's not a lager but the warmest yeast strain I know of is Wyeast 3724 (Belgian Saison) which is quite happy in the temperatures you describe.

An inexpensive chest freezer and a temperature controller would allow you to do a traditional lager and you will have excellent control over temperature. You can get chest freezers small enough to tuck into a closet now a days.

I think Bohemian Lager yeast is the one you can use the hottest. Alternatively, make a Kölsch (2565 yeast), that looks like a lager, but is in fact an ale :)

You can't brew a lager at room temperature.

But if you are asking how to brew a lager without a fridge in a room at ambient temperatures that's different.

You have to be very diligent but you can submerse your fermentor in a larger container/bin of water. Filling the exterior bin up with some water to cover most of the sides of the fermentor with water but not cause the fermentor to float. Regularly swap out frozen ice packs or water bottles or actual ice (if you can drain the the excess water out of the larger bin). Draping a towel over the fermentor and into the water somewhat and employing a fan will enhance the evaporative cooling effect.

The only problem with this method is that its labor intensive and doesn't afford a lot of control. But it can get you close to a pseudo lager. Using a "warm" fermenting lager yeast like WY2112 (California lager) helps.

Lastly, I don't know what your sources are, but reducing yeast cell counts is never a good idea and doesn't control for ester production very well. As others have said... it does the opposite.

Almost certainly No. 30C will kill most of your lager yeast, they are not very prolific above about 25C.

At a 30C room temp you would be best of trying to make a full flavoured fruity Belgium with banana aromas bursting forth, using a good Belgian strain.

I agree with @jsled you would be best off getting a small fridge and placing your fermenter in there. If that is not possible then evaporative cooling will drop your carboy a few degrees but not enough for a lager.

Perhaps, but I don't know how.

If you reduce the amount of yeast, you'll only create more off-flavors as the now-stressed yeast need to use all their metabolic reserves up in reproduction, rather than fermentation.

Trying to reduce the time of fermentation doesn't really work that way. Yeast are going to produce most of their flavor compounds (and off flavors) during the first few days of reproduction and fermentation. Trying to artificially shorten fermentation might even be a net negative, if you cut into the diacetyl rest while the warmer yeast is cleaning up byproducts from earlier in the fermentation.

As some have said, there are yeast that are better suited to fermenting in that temperature range, but are not really lager yeast. That's probably the best approach, apart from a small chest freezer to control fermentation temps.